Patrick McFarland, the inspector general of the Office of Personnel Management, confirmed to a Senate subcommittee Thursday that his office has been investigating USIS, the government’s largest contractor for background-investigation services, since late 2011. He said at least 18 security clearance investigators have been convicted of falsifying investigations since 2007. McFarland said there may be “considerably more” fraud that hasn’t been uncovered due to “alarmingly insufficient oversight” of the security-clearance process.

The same company that performed National Security Agency leaker Edward Snowden’s background investigation also performed a check of Aaron Alexis, the IT contractor who shot and killed 12 people at the Washington Navy Yard Monday. The Office of Personnel Management said it believes Alexis’ background check was complete and that the Defense Department signed off on the results of the background check.

The Justice Department has joined a whistleblower False Claims Act suit against the federal government’s largest provider of background investigations. Filed under the False Claims Act, the suit alleges that USIS, which currently has a multimillion-dollar contract with the Office of Personnel Management, failed to properly review its casework before providing it OPM.

The Justice Department has accused the company that performed background investigations of both National Security Agency leaker Edward Snowden and Navy Yard shooter Aaron Alexis of defrauding the government, making false statements and breach of contract. DoJ’s civil complaint filed Wednesday in U.S. District Court in Alabama alleges the company, which is the government’s largest contractor for background-investigation services, submitted at least 665,000 background investigations to the Office of Personnel Management that hadn’t been properly reviewed.

OPM Director Katherine Archuleta called Justice’s case an “egregious violation of the public trust.” OPM has taken steps to reform and improve the oversight of the security clearance system to prevent future “dumping” of cases.

Among the issues considered Tuesday by the House Oversight and Government Reform Committee were contracting practices at the Office of Personnel Management that allowed the largest background-investigation contactor — accused by the Justice Department of taking improper shortcuts and defrauding the government — to conduct quality reviews of its own work.

Less than two weeks ago, a federal judge approved the transfer of the case alleging USIS with improperly conducting thousands of background-check reviews to Washington, D.C. An investigation conducted by the Office of Personnel Management’s inspector general remains ongoing. OPM says it has confidence in the reforms put in place by the company.

Since 2008, the Office of Personnel Management has been on a crusade to root out falsification in background investigations using the courts. Nearly two dozen background investigators for either OPM or one of its contractors have been criminally prosecuted for misconduct ranging from outright falsifying reports, known as “ghostwriting,” to performing sloppy checks that failed to adhere to OPM’s standards.